One common communication medium employs frequency division multiplexing.
In frequency division multiplexing, a signal from an input channel is usually shifted in frequency by a transmitter modulator. The shifted signal then occupies a different portion of the frequency band, the band having been allocated for transmitting signals over a communication path between the transmitter and a receiver. For example, U.S. Pat. No. 3,697,682; issued Oct. 10, 1972 and entitled "Video Signal Processing"; discloses a method and apparatus according to which fields of a video frame are frequency interleaved. Each frame comprises a succession of P fields. Each field represents a different group of scan lines and thus each field includes a plurality of scan lines. Also, each field is of equal field scan time duration T.sub.f. In scanning a frame, each field of the frame is delayed by a time equal to the scan time T.sub.f multiplied by a factor (p-i) where i represents the ordinal place of the field in the succesion of fields which comprise the frame, i.e., i=1, 2, . . . , P. Thus, the first field of the frame is delayed a time T.sub.f multiplied by (P-1) while each succeeding field of the frame is delayed for a delay time equal to the delay time of the preceding field less the time of one field scan. Thereby, the respective fields coexist in time. Thereafter, each of the P fields is frequency shifted by a sinusoid having a frequency for shifting each field a different submultiple of the scan line frequency. As a result, the bandwidth of the resultant frequency multiplexed signal is essentially the same as that of the original video signal.
Another common communication medium employs time division multiplexing.
In time division multiplexing, the signal from an input channel modulates a train of pulses that are interleaved with the pulse trains of other channels. For example, the Bell System T1 carrier system interleaves an input signal from each of 24 input channels in a respective one of 24 time slots of a resultant 1.544 megabits per second time multiplexed signal. With such pulse code modulated (PCM) TDM arrangements as the T1 system, it is common for each time slot to include a digital representation of an analog input signal, the analog signal having been properly quantized according to some encoding law. However, it should be clear that a signal other than a quantized analog signal may be inserted in a time slot.
Thus, in frequency division multiplexing, the signal from each channel uses only a fraction of the bandwidth of the communication path but occupies the bandwidth all of the time. On the other hand, in time division multiplexing, the signal from each channel occupies the communication path only for a fraction of the time, i.e., during its time slot, but during that fraction of time the whole bandwidth is available to the signal.
Another form of multiplexing is called time compression multiplexing.
In time compression multiplexing, the signal from each input channel is stored for a short period of time. The signals from all channels are then read from the store, compressed in time and transmitted over the communication path. See, for example, the concurrently filed application of B. G. Haskell, Ser. No. 126,422 "Time Compression Multiplexing of Video Signals" in which a signal processor for time compressing video signals is disclosed. The signal processor includes a plurality, e.g., N, of input terminals, each input terminal adapted to receive a video signal v.sub.i (t), the video signal including a plurality of scan lines, and a scan line having a predetermined time duration, e.g., T seconds. The signal processor time compresses the video signal from a scan line time interval (O, T) by a predetermined compression factor M, e.g., M=N, to a time interval (O, T/N). Or the signal processor compresses the video signal v.sub.i (t) from the i-th input terminal, i=1, 2, . . . , N, to a time interval ##EQU1## There are N such intervals in the scan line interval (O, T). More specifically, the signal processor for time compressing the video signal includes apparatus for modulating the video signal and for extending the modulated signal through a respective first, predetermined delay network and, after adding delayed signals, for introducing a second, variable delay to the sum of the modulated signals whereby a time compressed video signal obtains. The modulated signal obtains by multiplying the video signal with a chirp signal. The second, variable delay obtains by extending the modulated signal through a dispersive filter. The time compressed video signal obtains by being extracted from the dispersive filter output by an envelope detector and being extended to a receiver where the compressed signal is expanded.
However, in time multiplexing the compressed signal, the compressed signal may need to be synchronized with an appropriate time slot before being extended to the receiver.